: Repeated treatment with methylphenidate (MPD; Ritalin) and other psychomotor stimulants, such as amphetamine and cocaine, has been shown to cause behavioral sensitization in rats and humans. Behavioral sensitization is characterized by an increase in locomotor and/or stereotypic behavior. It is not known, however, whether the behavioral sensitization produced by MPD involves the same neuronal mechanisms as amphetamine and cocaine. More importantly, chronic administration of MPD may also result in cross-sensitization with other psychostimulants. Methylphenidate is widely used to treat attention deficit/hyperactivity disorder (ADHD). The objectives of this study are to determine whether early exposure to MPD in juvenile rats increases their sensitivity to the drug when they are adult rats and whether treatment with MPD in juvenile and adult rats produces cross-sensitization to amphetamine. The study will consist of (1) monitoring the locomotor activity of two strains of rats (a model of ADHD and its control) before and after acute and repeated MPD administration when the rats are young and as adults followed by an amphetamine treatment and (2) evaluating sensory evoked potentials before and after drug treatment in freely moving rats with electrodes implanted in brain regions believed to be involved with ADHD, behavioral sensitization, and cross-sensitization.